There are many variations of this saying, another common saying relating to the topic is:
You can’t make good wine with bad grapes
All of these sayings really have one theme in common. High quality wine is made from high quality grapes. Anyone can grow grapes but, not everyone can grow high quality grapes. Not everyone can make high quality wine.
As any winemaker the secret to making great wine and they will take you out in the vineyard. They will talk about the weather that season, how they are monitoring it day by day. They will talk about the adjustments that they are making based on these weather conditions.
When it comes to the actual making of the wine (crushing, pressing, fermenting, aging, bottling), everyone is just following a process. They are using techniques and executing them perfectly. However, even if the best winemaker is executing everything perfectly in the production facility if the starting juice is sub-par then at best, the wine will be sub-par.
Wine is made in the vineyard.